Monstrous Compendium Vol. 1: Spelljammer Creatures
Frozen Sick
D&D Beyond Basic Rules
Basic Rules (2014)
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Monstrous Compendium Vol. 1: Spelljammer CreaturesFrozen SickD&D Beyond Basic RulesBasic Rules (2014)Sign in to view your library & saved favorites.
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Monster stat blocks don’t “level up”. If it were intended that the spell be cast at level 9, it would say so.
That said, if you’re the DM, you can have it work however you want.
pronouns: he/she/they
Thank You for pointing that out, so it is static unless DM says so, good to know
The implication is not that the dragon has somehow "leveled up" their spells. Dragons are creatures of innate magic, it's why one of the sorcerous bloodlines is draconic. As they grow older, their innate connection to magic deepens, increasing their access to newer and more powerful magic. So an Ancient Red Dragon would have access to more powerful magic than an Adult Red Dragon. Because their magic is innate, the nature of it is fairly static - red dragons tend to favor magic that is more aligned with domination and destruction, while silver dragons tend to favor magic that is more aligned with control of the physical form - so they'll develop magic that is more aligned to their particular axiom of power. It's not outside the realm of common sense to say that, as they grow into something like a "super-ancient," they might further increase the levels of the spells they have access to and/or deepen their pool of available magic, but that'd be something you'd have to account for on your end.
To the point of your original post, when talking about how having a magic item would affect the CR of a creature, I really wouldn't consider dragons as eligible for being affected by the presence of a magic item. Dragons are natural hoarders of objects of power, and to assume you'd wander into a dragon's lair (especially an ancient dragon's lair), and there wouldn't be a vast array of magic items that the dragon has at its disposal would be the laziest of missteps. CR is a measure of a creature's ability to challenge a party - specifically, it measures the level a party of four adventurers would have to be for a creature to cost 25% of their resources to defeat. One of those resources should be a player's life. So a CR24 creature would take a party of four twenty-fourth-level adventurers to take down with a relative degree of safety. A party of 20th-level players should be at a very high risk of a TPK unless they're rocking some tremendous late-game rewards. The reality is, the Ancient Red Dragon's stat block is simply not impressive enough for that to be true unless they're utilizing traps, henchmen, and an array of carefully cultivated, powerful magic items to assist them in the fight. DM's who have you fight an Ancient Red Dragon and just use its fire breath and claws, then when you win roll a horde that has, like, six legendary items in it are doing their monsters a disservice. So whatever your item is, assume the dragon using it is part of it's CR, and plan accordingly.
I will say, if you're a self-described "new DM," and you're considering building this monster for an actual fight, you might want to... not? High-level combat is complicated and even those of us with decades of experience avoid it when we can. If you're just making this creature to be an antagonist that pokes at the players from the shadows, that's well and good, but if you're running a campaign with 20th-level players planning to fight a red dragon, there are a lot of things to keep track of. And especially with a red dragon, there are a lot of things to keep track of. An Ancient Red Dragon will know your players are planning to fight them before the players do. They'll have built an elaborate and complex dungeon specifically designed to strip the players of any preparatory magic and items they might bring to the fight, they'll pick servants that play to the players' weaknesses, and access magic that will counter the players' strengths. The dragon will have agents whose entire goal is to spread misinformation about the dragon's own strengths and weaknesses, and the dragon will know what the players are planning as the players are planning it. If the players are planning on taking on the dragon, they'll need to disrupt the dragon's supply and intelligence networks, counter its Scrying every day as they plan, build their own network of trusted allies to gather reliable information, and then they'll need to doubt every scrap of information that they gather. A red dragon will specifically look to turn the parties' allies, either through brute magical force, intimidation, or extortion, to the point that the players can't trust their own best friends. And, should the players somehow manage to make it to the dragon's lair, and get even remotely close to putting the dragon in even the slightest bit of danger, a red dragon will flood the room with lava and fly away, destroying every scrap of treasure in their hoards, just to deny the players the satisfaction of having it for themselves.
All that, and you'll still have to run an encounter with a creature that, on its own, could probably take down at least one party member before fleeing.